Is the Glycemic Index Useful?

Introduction: The glycemic index was developed to inform us about the degree to which carbohydrate foods raise blood sugar. Several popular reduced-carbohydrate diets, such as the South Beach Diet use the glycemic index. Yet, there are contradictory and somewhat confusing issues regarding using these numbers when choosing specific foods to eat.

Why is Knowing the Glycemic Value of Foods Important?: A lot of reliable research shows that eating a diet with a relatively low blood glucose impact can protect us from diabetes Dietary fiber, glycemic load, and risk of non-insulin-dependent diabetes mellitus in women, and likely from heart disease A prospective study of dietary glycemic load, carbohydrate intake, and risk of coronary heart disease in US women as well. Therefore, choosing foods that are less likely to rapidly raise our blood glucose can be a healthy way to eat, especially for those prone to insulin resistance, diabetes, and heart disease.

How is the Glycemic Index Calculated?: A group of people will eat a food with a standard amount of carbohydrate, usually 50 grams. Then their blood is tested every 15 minutes to determine how much and how fast their blood sugar rises. The higher and faster it rises, the higher the score. The index assigns foods a number on a scale of one to 100, in comparison with pure glucose, which has a reference score of 100. Under 55 is considered to be a “low” GI, above 70 is “high” and in between is “medium”.

What are the Problems with the Glycemic Index?: Although it would indeed be extremely useful to have a good measure of how glycemic various foods are, the glycemic index has several problems. The most important ones are: the small number of foods tested, the small range of GI measurements, the variability of foods tested, and the fact that the glycemic index doesn’t simulate the way people actually eat.Information about the testing and the values of individual foods taken from International table of glycemic index and glycemic load values: 2002

Not Many Foods Have Been Tested: The main foods tested for the glycemic index are those high in carbohydrates, because of the need to eat enough of a food at one time to get 50 grams of carbs. So, one could reasonably eat 1 and ¼ cup of spaghetti, or 3 cups of peas. But to get 50 grams of carbs from broccoli you’d have to eat somewhere between 16 and 22 cups of chopped broccoli. For cheddar cheese, try 9 pounds of cheese. So foods with lower amounts of carb were not tested for the glycemic index.

The Range of the Glycemic Index Values is Small: Two foods have a GI over 90 – parsnips and amaranth. Above 80 are potatoes, honey and some processed cereals. The 70’s have Gatorade, white bread and rice, watermelon, and some processed baked goods such as donuts. At the low end of the scale, the foods with GI values under 40 are various beans, grapefruit, peaches, and nuts. By far most of the foods tested (which is a small percentage of total foods) have GIs between 40 and 70. This is even more of an issue because of the next problem:

Many Foods Have a Lot of Different Glycemic Index Values: Different recipes, varieties of plants, and ways of preparing foods yield different values, as do different laboratory techniques (the data was gathered from all over the world). There are different listings for various cakes with GIs from 38 to 87. (Why do beans have GIs below 40, whereas bean soup is listed at 64?) When you combine that with the small range of values, and the differences between individuals, it is difficult to know what the real differences between foods are.

We Don’t Eat the Way the Glycemic Index was Obtained: We don’t eat one food at a time. If we eat a lot of different carbohydrate foods in a meal, how do we count it? Protein and fat tends to lower the glycemic index in a meal, but we have no way of knowing how much, short of each person testing their own blood.

Enter the Glycemic Load: The glycemic load was invented to account for serving size, since we seldom eat exactly 50 grams of carbs from any one food. Theoretically, glycemic load should resolve some of the problems inherent in the glycemic index. However, one of the basic problems with the glycemic load is that it is based on the glycemic index, so it has many of the same difficulties – not many foods tested, much variability, etc.

Problems with the Research: Although many studies have shown value in using the glycemic index, others which haven’t shown positive results have had similar problems. For example, in a recent study, researchers essentially guessed at GI values for foods not on the list (e.g. giving cheese the same GI as milk). This research also shows the problem with range – very few people in the study ate a diet classified as low GI – they were almost all eating a diet with quite a high glycemic index or load, so it isn’t surprising that results weren’t conclusive. The upshot is that we must be very careful when looking at the research on these topics.

So, What Should We Do?

There is no doubt that eating a low glycemic diet is a Good Thing. And the research on the glycemic index does give us a few clear guidelines, such as to minimize potato consumption. However, I think at this stage of the game, it is a better idea to just go with total amount of carbohydrate. Carbohydrate does raise blood glucose, and insulin must be released to stabilize it. The easiest way to limit the problems caused by high glycemic foods is simply to follow a reduced-carbohydrate diet.